Prediction of Solidification Paths in Al-Si-Fe Ternary System and Experimental Verification: Part I. Fe-Containing Hypoeutectic Al-Si Alloys

Abstract

The effects of Fe content and cooling rate on the solidification path and formation behavior of the Al5FeSi (β) phase in Fe-containing hypoeutectic Al-Si alloys were studied based on thermodynamic analysis and pertinent experiments. The thermodynamic calculations were performed using the Thermo-Calc program. For analyses in the high alloy region of the Al-Si-Fe ternary system, a thermodynamic database for Thermo-Calc was correctly updated and revised by the collected up-to-date references. For thermodynamics-based predictions of the solidification path in Fe-containing hypoeutectic Al-Si alloys, liquidus projection (including various invariant, monovariant, and bivariant reactions and isotherms) and equilibrium phase fraction were calculated as functions of composition and temperature in the Al-Si-Fe ternary system. The calculated results were compared to experimental results using various casting runs. In order to analyze the solidification path as a function of Fe content, two representative hypoeutectic Al-Si alloys with different Fe levels were designed. To better understand the influence of cooling rate on the formation behavior of the β phase, the two alloys were solidified under slowly- and rapidly-cooled conditions, respectively. The cooling curves of the solidified alloys were recorded by thermal analysis and various important solidification events were detected using the first derivative of the cooling curves. Microstructures of the casting samples were studied by combined analyses of optical microscopy (OM) and scanning electron microscopy (SEM). For the slowly-cooled condition with the high Fe level, the primary β phase enveloping the Al8Fe2Si (α) phase was mainly formed by the quasi-peritectic reaction of L + α → (Al) + β (612°C). For the rapidly-cooled condition with the high Fe level, the primary β phase with morphology of a curved needle was mainly formed by the reaction of L → (Al) + β (579∼612°C).